The gas-liquid contacting device of a flotation cell usually has two purposes: to disperse the gas throughput into extremely small primary gas bubbles; and to create a field of shear stresses which causes the solid particles of the pulp to attach themselves to the gas bubbles. Stirrers embodying the rotor/stator principle are used preferentially for these purposes because they produce a high energy dissipation density within a very small space, but cause relatively little motion of the liquid outside the stator. The latter is necessary to ensure that the solid particles and gas bubbles, which become attached to one another in the mixing chamber within the stator ring, are not separated from one another as they rise to the surface.
Stirrers embodying the rotor/stator principle are described in, e.g., Ullmann, Encyklopadie der techn. Chemie, Vol. 2, 1972, and are either so designed that they suck in the gas themselves or receive the gas from a special gas-supplying device. As the rotational speed of the stirrer cannot be varied as desired under ordinary industrial conditions, the gas throughput is the only freely selectable process parameter that can be used to adjust the gas dispersing device to the flotation process. This lack of flexibility of the process parameters is undoubtedly a disadvantage of stirrers embodying the rotor/stator principle.
A further disadvantage of stirrers embodying the rotor/stator principle is the relative complexity of their design, which makes them very prone to wear. The reliability of many types of stirrer is also reduced by the fact that the bearings of the stirrer shaft are inside the pulp tank.
The aforementioned disadvantages of these gas dispersion devices for flotation cells presumably explain why efforts have been made for a number of years to develop what can be called a "stirrerless" flotation cell, cf., e.g., P. John, Aufbereitungstechnik 5 (1964) 10, 532-543. The reason why such cells have established themselves in exceptional cases only, is undoubtedly connected with the fact that the alternative gas dispersers, though able to produce fine gas bubbles, are unable to produce the shear stress field needed to couple the bubbles and solid particles, or they circulate the liquid in the cell so vigorously that the contact between the bubbles and solid particles is destroyed as the bubbles rise towards the surface.
Due to the last reason, the usual two-phase nozzles, for example, injectors, ejectors, Venturi nozzles, and jet nozzles, as described in DAS No. 2,458,449, DAS No. 2,166,595, DOS No. 2,410,570, DOS No. 2,408,064, and DOS No. 2,634,494, are unsuitable as gas distributors for flotation cells. They emit the gas/liquid dispersion as a powerful free jet which, though favourable to the mass transfer between the gas phase and liquid (absorption process), causes pronounced turbulence in the surrounding liquid.